The present invention relates to a cold isopressing method in which material is compacted within an isopressing mold. More particularly, the present invention relates to such a method in which two or more layers of material are formed within an isopressing mold and the second of the layers is isostatically pressed against the first of the two layers to compact the second layer.
Cold isopressing is a well-known technique that is used to form filters, structural elements and ceramic membranes. In cold isopressing, a granular form of a material to be compacted is placed within an elastic isopressing mold that is sometimes called a bag. The granular material can be a ceramic or metallic powder or a mixture of powder, binder and plasticizing agents.
The isopressing mold is then positioned within a pressure vessel and slowly subjected to a hydrostatic pressure with either cold or warm water to compact the granular material into a green form which subsequently, as appropriate, can be fired and sintered.
The isopressing mold can have a cylindrical or flat configuration to produce cylindrical or plate-like articles, respectively. An example of such a process that is applied to the formation of tungsten rods is disclosed in U.S. Pat. No. 5,631,029. In this patent, fine tungsten powder is isostatically pressed into a tungsten ingot.
An important application for ceramic materials concerns the fabrication of ceramic membrane elements. Such ceramic membrane elements are fabricated from a ceramic that is selected to conduct ions of either oxygen or hydrogen at high temperatures. In an oxygen-selective membrane, the heated membrane is exposed to an oxygen-containing gas that ionizes at a cathode side of the membrane. Under a driving force of a differential oxygen partial pressure, oxygen ions are transported through the membrane to an opposite anode surface. The oxygen ions combine at the anode side of the membrane to give up electrons that are transported through the membrane or a separate electronic pathway to ionize the oxygen at the cathode side of the membrane.
A recent development in ceramic membrane technology is to form a thin dense layer of material on a porous support. The dense layer conducts ions and the supporting structure functions as a percolating porous network to add structural support to the dense layer. The porous support may also be fabricated from a material that is itself capable of transporting ions so as to be active in separating the oxygen.
Ceramic membranes such as have been described above, may be in the form of plates or tubes. It is difficult, however, to impart a complex architecture to such membranes. In the manufacture of composite tubular structures, the tube is formed by a process such as slip casting or extrusion and sintered. Thereafter, a dense layer can be sputter deposited on the outside of the extrusion. In U.S. Pat. No. 5,599,383, the dense layer is applied by chemical vapor deposition. In order to produce an even more complex architecture, several different types of processing techniques must be applied. It is desirable, however, that the number of processing steps, be minimized in that ceramic materials are, by their very nature fragile.
As will be discussed, the present invention provides a cold isopressing method in which complex structures may be directly formed without the type of complex processing stages that have been used in the prior art.
The present invention provides a cold isopressing method in which at least first and second layers are isostatically pressed within an isopressing mold so that at least the second layer is compacted and the first and second layers are laminated. The first and second layers can be formed of two different materials. For instance, the first layer could be a metal tube or other pre-form and the second layer could be a ceramic slurry coating on the tube. After the isostatic pressing, the ceramic particles within the slurry would be compacted. Alternatively, the first layer could be a granular material for instance a metallic or ceramic powder that is compacted within the isopressing mold. The resultant compacted element could then be coated with a slurry to form a second layer or the second layer could be another granular material to be compacted against the first layer. Other layers could be added or the compacted form could be further processed into a finished article. For instance in case of ceramic materials, the compacted form could be subjected to firing to burn out organic materials, such as binders and plasticizing agents, followed by sintering to produce the finished article. Alternatively, the first and second layers can be formed of the same material, for instance, if a thick ceramic article were desired, the first layer containing the material in granular form could be compacted in a cylindrical isopressing mold. Thereafter, a second layer of the same material could be placed within the isopressing mold and compacted to begin to form an additional thickness. A further possibility is to form at least one of the first or the second of the layers with at least two regions containing different materials. It is to be noted that the term xe2x80x9cgranular formxe2x80x9d as used herein and in the claims to mean either a powder or a powder mixed with other agents such as binder or plasticizing agents.
An isopressing mold that can be used to form a tubular structure, such as required for a tubular ceramic membrane element, can be provided with a mandrel coaxially located within the cylindrical pressure bearing element to form a tubular structure. The first layer is formed about the mandrel and the second layer is formed about the first layer. The resultant tubular structure could be a tubular ceramic membrane element of the type described above. In this regard, the two layers can consist of green ceramic materials such as ceramic powders or ceramic powders mixed with other agents such as plasticizers, binders and etc. or one of the two layers could be in the form of a ceramic containing slurry. Specifically, the first layer could be formed by introducing the first of a green ceramic materials in granular form into the isopressing mold and then isostatically pressing the first of the green ceramic materials. Thereafter, the second layer could be formed by isostatically pressing the second of the green ceramic materials in granular form onto the first of the layers.
In an isopressing mold, such as has been described above, the first green ceramic material is introduced into an annular space between the mandrel and a first cylindrical pressure bearing element for isostatic pressing. After the formation of the first layer, the first cylindrical pressure bearing element can be removed and a second cylindrical pressure bearing element, having a different diameter than that of the first cylindrical pressure bearing element can be coaxially positioned over the first of the at least two layers to form another annular space. The second of the green ceramic materials is introduced into this other annular space in granular form for isopressing and formation of the second layer.
An alternative manner of forming the first layer is forming a dry slurry coating on the mandrel which has been coated with a suitable release agent, the slurry containing the first green ceramic material. The second of the two layers can then be formed by isopressing a second of the green ceramic materials in granular form against the dry slurry coating. Alternatively, the first layer can be formed by introducing a first of the green ceramic materials in granular form into the isopressing mold and isostatically pressing the first of the green ceramic materials. The second layer can then be formed by forming a dry slurry coating on the first layer, the dry slurry coating containing a second of the green ceramic materials. Thereafter, the second of the green ceramic materials is isopressed against the first of the green ceramic materials.
In order to form still more complex architectures, channel-forming elements can be positioned between the layers. Such channel-forming elements can be formed from paper or other pyrolyzable materials that will burn out during firing to produce the channels between the layers.
Another alternative is to provide one or more of the green ceramic materials in granular form with pore formers. Such pore formers might, for example, be starch, graphite, polyethylene beads, polystyrene beads or sawdust. Thus, a thin ceramic layer could be formed on the inside of a tubular membrane by, for instance, a slurry coating on the mandrel. Thereafter, the porous support layer could be formed by a green ceramic material containing the pore formers. After firing, the pore formers would burn out to leave the pores. In this regard, preferably the pore formers are present within the green ceramic materials in amounts sufficient to produce a porosity of between about 1% and about 90% after firing.
The first layer can be formed by extrusion, slip casting, dry pressing or injection isopressing molding. Thereafter, the second layer in granular form or in the form of a slurry can be introduced into the isopressing mold and isostatically pressed against the first layer.
As may be apparent, additional layers containing the same or different materials can be added to form a variety of porous or dense layers. Furthermore, at least one of the first and the second layers can be formed from at least two levels of different green ceramic materials.
At least one of the green ceramic materials can be a mixed conducting oxide given by the formula: AxAxe2x80x2xxe2x80x2Axe2x80x3xxe2x80x3ByBxe2x80x2yxe2x80x2Byxe2x80x3O3xe2x88x92z, where A, Axe2x80x2, Axe2x80x3 are chosen from the groups 1, 2, 3 and the f-block lanthanides; and B, Bxe2x80x2, Bxe2x80x3 are chosen from the d-block transition metals according to the Periodic Table of the Elements adopted by the IUPAC. In the formula, 0 less than xxe2x89xa61, 0xe2x89xa6xxe2x80x2xe2x89xa61, 0xe2x89xa6xxe2x80x3xe2x89xa61, 0 xe2x89xa6yxe2x89xa61, 0xe2x89xa6yxe2x80x2xe2x89xa61, 0xe2x89xa6yxe2x80x3xe2x89xa61 and z is a number which renders the compound charge neutral. Preferably, each of A, Axe2x80x2, and Axe2x80x3 is magnesium, calcium, strontium or barium.
As an alternative, at least one of the green ceramic materials can be a mixed conducting oxide given by the formula: Axe2x80x2sAxe2x80x3tBuBxe2x80x2vBxe2x80x3wOx where A represents a lanthanide, Y, or mixture thereof, Axe2x80x2 represents an alkaline earth metal or mixture thereof; B represents Fe; Bxe2x80x2 represents Cr, Ti, or mixture thereof and Bxe2x80x3 represents Mn, Co, V, Ni, Cu or mixture thereof. Each of s, t, u, v, and w represent a number from 0 to about 1. Further, s/t is between about 0.01 and about 100, u is between about 0.01 and about 1, and x is a number that satisfies the valences of A, Axe2x80x2, B, Bxe2x80x2, and Bxe2x80x3 in the formula. Additionally, 0.9 less than (s+t)/u+v+w) less than 1.1.